1. Field of the Invention
The invention relates to a method and an apparatus for controlling the nozzle engagement of an injection molding unit in electrically driven injection molding machines.
2. Description of Related Art
While nozzle engagement and pressing the nozzle into engagement with the mold are among the most important parameters in an injection molding machine, as regards the injection operation they are actually one of the less relevant ones. There is a demand for a rapid and an as careful as possible nozzle engagement at a large engagement force, so that no melt escapes between the injection nozzle and its injection port at the extremely high injection pressure of 2,000 bar and more. Escaping material directly influences the operational precision and causes an annoying waste of plastic. In an extreme case, it may also lead to an interruption of the automatic injection operation, for the tightness is reduced further with an increased escape of material. The aim of the nozzle engagement is to find an optimum as regards speed of movement while preventing an abrupt engagement and unnecessarily large engagement forces. If the nozzle engagement force is too large, it may even lead to mechanical damage of the mold or the mold support.
As far as is known to the applicant, to optimize the entire nozzle engagement operation in a practical manner, a mechanical compression spring or an entire spring system is mounted between the drive and the slide mechanism of the injection unit of all conventional electrically driven injection molding machines. In such arrangements the spring serves several functions, so that the nozzle engagement force is be controllable by electric motor drives. For instance, European Patent No. 328,671 proposes to prestress a double spring package to a greater or lesser degree, so that the corresponding force may be preset for varying engagement forces. To generate the actual nozzle engagement force, an additional spring play of up to several millimeters is tolerated. This play has to be compensated by appropriate open and closed loop controls by the drive motor. The spring package renders the entire system resilient, which, on the one hand, yields advantages, but, on the other hand, leads to an enormous complication of the control technology for an actually very simple operation. In terms of the operation, the only requirement is to maintain the nozzle in tight engagement with the injection port.
The subsequent European Patent No. 422,224 attempts to overcome this disadvantage by providing additional power sensors or strain gages, as the case may be, in the vicinity of the injection unit. A predetermined nozzle engagement force is preset, and thereafter the drive motor is controlled by comparing the measured force against the preset force, or on the basis of the deviation between the desired and actual forces. Even though the control technology per se could be simplified, the overall system remains complicated, may be prone to malfunction, and is rather complex especially as regards its structure.